1. Field of the Invention
This invention relates to a process for the preparation of thermoplastic polyurethane elastomers.
2. Prior Art
Thermoplastic polyurethane elastomers have been known for a considerable length of time. Their technical importance is based on a combination of high grade mechanical properties, together with the advantages of costeffective thermoplastic processing methods.
By using varying chemical components, a great variety of mechanical properties can be achieved. An overview of thermoplastic polyurethane elastomers, their properties and applications, is given in Kunststoffe 68 (1978), pages 819-825. Thermoplastic polyurethane elastomers may be prepared continuously or on a batch-type basis, employing various methods. As well known, the so-called band method and the extrusion processes are used on an industrial scale also.
According to British Patent No. 1,057,018, a prepolymer is produced from a basically linear polyhydroxyl compound and excess organic diisocyanate. Via metering pump this prepolymer is introduced into a mixing head where it is mixed with a certain amount of a low molecular diol. The resultant reaction mixture is fed onto a conveyor band and is transported through an oven heated to a temperature of 70.degree. C. to 130.degree. C. until it solidifies. The reaction product is subsequently subjected to a size reduction process and is tempered at a temperature up to 120.degree. C. for periods of 6-40 hours. Thus the product can be processed into molded parts, for example, using injection molding machines. The post-tempering and size-reduction processes have a detrimental economical effect on the band method.
In the case of the extruder process, such as described for example in German Published Application No. 20 59 570 (U.S. Pat. No. 3,642,964), the components are directly introduced into the extruder and the reaction is carried out in the extruder under certain process conditions. The resultant polyurethane elastomer is transformed into a thermoplastic state, is extruded, is cooled to solidification in an inert gas atmosphere and is subjected to a size-reduction process. The drawback of this process is that the resultant thermoplastic polyurethane elastomers are not suited for the preparation of films or of fine profiles and hoses. Thermoplastic polyurethane elastomers of the same composition are transparent when prepared according to the extruder method, whereas they are opaque if produced according to the band method. Opaque thermoplastic polyurethane elastomers can be processed into films which do not display any blocking, whereas transparent thermoplastic polyurethane elastomers are not suited for this application. The drawback of the band method, however, is the cost.